Potential transformer

ABSTRACT

An apparatus used in measuring the potential of an external source utilizing inductively coupled high and low voltage windings. The high voltage winding is connected to the source being measured through an electrical bushing assembly. The bushing assembly comprises stress grading shields which are terminated by stress rings.

United States Patent Conner et al.

1451 Aug. 22, 1972 [54] POTENTIAL TRANSFORMER [72] Inventors: Edmond E. Conner, Brookfield, Ohio; Edward C. Wentz, Sharon,

Hoffman l 74/ l 27X 3,299,383 1/1967 Conner et a1. ..336/84 X 3,173,115 3/1965 Peuron ..336/70 X FOREIGN PATENTS OR APPLICATIONS 612,061 0/1948 Great Britain l74/142 1,127,427 4/1962 Germany 174/73 882,379 11/1961 Great Britain ..336/84 602,394 9/1934 Germany ..336/ 84 Primary Examiner-Thomas .l. Kozma Attorney-A. T. Stratton and F. E. Browder 1571 ABSTRACT An apparatus used in measuring the potential of an external source utilizing inductively coupled high and low voltage windings. The high voltage winding is connected to the source being measured through an electrical bushing assembly. The bushing assembly comprises stress grading shields which are terminated by stress rings.

10 Claims, 5 Drawing Figures 1 It I I gl igziaaaaeelllllll FIG. 2

Patented Aug. 22, 1972 2 SheetsSheet 1 POTENTIAL TRANSFORMER BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates, in general, to electrical inductive apparatus and more particularly to potential type instrument transformers.

2. Description of the Prior Art The physical size of high voltage electrical inductive apparatus, such as potential transformers, is determined mainly by the insulation requirements rather than by the voltampere rating of the apparatus. Considerable size reduction in potential transformers immersed in a liquid dielectric, such as oil, has been accomplished by using a solid insulating material comprising fibrous material, such as crepe paper. Also, a large portion of the weight and cost of high voltage inductive apparatus is in the tank and the bushing for the primary lead. I

Condenser type bushings produce linear voltage distribution along their outer surfaces, but forreasons of use simple bulk-type bushings in instrument transformers and utilize other means for obtaining longitudinal stress control along the high voltage bushing. Some prior art methods use a potential stress grading system similar to that of a bushing, except that the step-located concentric shields are incorporated into the primary lead insulation rather than in the bushing itself. This method, however, has disadvantages in that the primary lead insulation must be made relatively thick and with a large plurality of shields in order to reduce the voltage steps and edge effect on the shield end. Corona will develop on the sharp ends of the shields if the voltage between adjacent shields is too great.

Another method uses a grounded stress shield having an hour-glass shape. The center of the shield is located at the opening in the transformer cover through SUMMARY OF THE INVENTION This invention discloses a new and improved electrical inductive apparatus of the type used for measuring high potential. The potential transformer includes a magnetic core and winding assembly which is housed in a metallic tank. The primary lead from the high voltage winding extends through a bushing assembly to an expansion cap to which the external potential source is connected. The bushing assembly includes a plurality of layers of solid insulation surrounding the high voltage lead and contained within the bushing insulator. Electrical conducting stress shields are contained within the solid insulation to lower the potential gradient within the insulation. The stress shields, which may exhibit severe gradient characteristics at their ends, are terminated by stress rings which reduce the potential gradient. The use of the stress rings permits reduction of the insulation thickness and physical dimensions of the potential transformer.

BRIEF DESCRIPTION OF THE DRAWING FIG, 1 is an elevational view, partially in section, with parts broken away for clarity, showing a potential type instrument transformer constructed according to the teachings of this invention;

FIG. 2 is an elevational view, in section, showing the bushing assembly of the potential transformer of FIG.

FIG. 3 is an elevational view, in section, showing a stress ring assembly constructed according to the teachings of an embodiment of this invention;

FIG. 4 is an elevational view, in section, illustrating an intermediate step in the construction of the stress ring assembly shown in FIG. 3; and

FIG. 5 is an elevational view, in section, showing a stress ring assembly constructed according to the teachings of an embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the following description similar reference characters refer to similar members in all Figures of the drawing.

Referring now to the drawing, and FIG. 1 in particular, there is shown an elevational view of a potential instrument transformer with parts broken away for clarity. A tank 10 with a cover 12 supports a bushing insulator 14. The bushing insulator 14 is constructed of an insulating material, such as porcelain or epoxy, and has adequate physical dimensions to provide sufficient creepage distance along its outermost surface. An expansion cap 16 with high voltage terminal 18 is connected to the top of the bushing insulator 14.

The tank 10 houses a winding assembly 21 and a fluid dielectric material 23, such as mineral oil. The winding assembly 21 comprises a magnetic core 20, a low voltage winding 22, a high voltage winding 28, and insulating material disposed on the high and low voltage windings. The insulating material may be crepe paper, Nomex, or any other insulating material which may be impregnated with the liquid dielectric. The insulating material is disposed onthe high and low voltage windings to provide electrical insulation in a region 24 between the magnetic core 20 and the low voltage winding 22, a region 26 between the high and low voltage windings, 28 and 22 respectively, and a region 30 between the high voltage winding 28 and the magnetic core 20, tank 10, and cover 12.

The magnetic core 20 is a three-leg laminated core with the winding assembly 21 disposed around the middle leg. The magnetic core 20 is diagonally positioned within the tank 10 in order that the physical dimensions of the tank may be minimized.

The high voltage winding 28 comprises a plurality of conductor turns concentrically disposed on the low voltage winding 22. A ground lead 32 of the high voltage winding 28 is brought through the tank 10 by a bushing 34. A high voltage lead 36 extends through an opening 38 in the insulation region 30. The high voltage lead 36 is connected to a high voltage lead extension 40 by a connector 42. The high voltage lead extension enters the expansion cap 16 at point 44 and is electrically connected to the metallic expansion cap by terminal 46. The low voltage winding 22 includes leads 48 to which external potential indicating apparatus or circuitry is connected.

The high voltage lead 36 and extension 40 are positioned along the axis of the bushing insulator 14. A stress grading insulation assembly 50 surrounds the high voltage lead 36 and lead extension 40. A more detailed illustration of the stress grading insulation assembly 50 is shown in FIG. 2.

FIG. 2 is an elevational view, in section, showing in detail the stress grading insulation assembly 50 and its position relative to the other members of the transformer. The bushing insulator 14 is mounted on the tank cover 12 by a suitable means, such as flange 51, ring 52 and adhesive 54. Similarly, the expansion cap 16 is mounted to the bushing insulator 14 by a suitable means, such as ring 56 and adhesive 58.

The insulation region 30, which is adjacent to the outermost surface of the high voltage winding 28, contains the opening 38 through which the high voltage lead 36 extends. The opening 38 has a circularly shaped cross section in the plane perpendicular to the bushing insulator axis. The diameter of the opening 38 is sufficient to permit penetration of the lower portion of the stress grading insulation assembly 50 into the insulation region 30, to a depth near the outermost surface of the high voltage winding 28.

An electrical conducting tubular member 60, having an upper or first end 62 and a lower or second end 64, is disposed around the high voltage lead 36 and lead extension 40. The tubular shielding member 60 is constructed of materials which provide adequate electrical conduction properties and sufficient mechanical rigidity. A metallic tube or a tube formed by winding kraft paper and aluminum foil may be used. The upper end 62 of the shielding member 60 is connected to the expansion cap 16 which places the shielding member at the same potential as that of the high voltage lead. The lower end 64 of the shielding member 60 projects into the opening 38 in the insulation region 30.

The lower end 64 of the shielding member 60 is terminated by a stress ring 66. The stress ring 66 has a circularly shaped cross section and is constructed of an electrically conductive material such as a metal rod or tube. The stress ring 66 may also comprise a tube or roll of crepe paper and aluminum foil. The material used to construct the stress ring 66 is bent to form a circular shape with approximately the same diameter as the shielding member 60. The stress ring 66 is positioned on or near the lower end 64 of the shielding member 60 with the axis about which the stress ring 66 is bent coinciding with the axis of the shielding member. Utilization of the stress ring 66 reduces the potential gradient at the lower end 64 of the shielding member 60 by effectively increasing the termination area of the conducting portion of the shielding member 60. The stress ring 66 may be attached to the shielding member 60 by a welding process if the materials used permit welding. The stress ring 66 could also be formed by placing a crepe paper tube around the conducting layer of the shielding member 60 and folding the conducting layer over the crepe paper tube. Using aluminum foil as the conducting layer is sufficient since negligable current flows in the conducting layer.

A suitable solid insulating material, such as crepe paper, is disposed circumferentially on the shielding member 60 forming insulation region 68 which is between the shielding member 60 and an intermediate shielding member 72. The lower portion 70 of the insulating material is folded around the stress ring 66 and into the lower portion of the shielding member 60. The thickness of the insulation in region 68 is dependent upon the class and rating of the transformer. The upper end 74 of the insulation region 68 extends a sufficient distance above the stress ring 76 to prevent electrical discharges between the stress ring 76 and the shielding member 60.

The intermediate stress grading shield 72 comprises an electrically conductive material and stress rings 76 and 78 at the upper end and lower end, respectively, of the conducting material. Although only one intermediate stress grading shield is illustrated in FIG. 2, more than one intermediate shield may be similarly disposed to provide adequate stress grading. The stress grading shield 72 may comprise a metallic tube or an insulating tube with a conductive coating disposed thereon. An electrically conductive material such as aluminum foil may also be wound on the crepe paper insulation of region 68 to provide the conduction properties. The details of the construction of the stress rings 76 and 78 are discussed hereinafter. The lower stress ring 78 is positioned outside the insulation region 30 but, due to the relatively low potential gradient surrounding the stress ring 78, it can be positioned close to the insulation region 30.

An outermost or grounded stress grading shield 80 is separated from the intermediate stress grading shield 72 by the solid insulating material in region 82. Crepe paper may be used as the insulating material in region 82 with the thickness thereof dependent on the class and rating of the transformer. The grounded shield 80 may be constructed of materials similar to that used for the intermediate shield 72, that is, a metallic tube, conducting coating or aluminum foil. The upper end of the grounded shield 80 is terminated by stress ring 84 and the lower end by stress ring 86. The lower stress ring 86 is positioned in or near the plane containing the tank cover 12. Since the ground shield 80 and its stress ring are essentially at the same potential as the tank cover 12, the thickness of the insulation in the region 88 is not critical. The stress shield 80 is electrically grounded by lead 90 which may be connected to the cover- 12 or flange 51 and the stress shield 80. If the stress shield 80 is formed by winding aluminum foil around the insulating region 82, the lead 90 can be connected to the aluminum foil by winding the lead around the foil several times. Solid insulation in the region 92 holds the lead turns against the aluminum foil to provide adequate physical contact. An outermost layer 94 of suitable material is wound around the stress grading insulation assembly 50 to secure the internal insulating and conducting layers. The location of the stress ring 84 is between the adjacent stress ring 76 and the tank cover 12, the exact location being established by the class and rating of the transformer.

Stress rings 76, 78, 84 and 86 may be constructed as shown in detail in FIG. 3 which is an elevational view, in section, of atypical stress ring. Solid insulation comprises regions 96, 98, and 102. Layer 104 comprises an electrical conducting material such as aluminum foil which is disposed around a tubular member 106. The tubular member 106 may be comprised of insulating material, such as crepe paper, or'conducting material, such as copper or steel, or any other suitable material.

Although other methods may be used, a convenient method of constructing the stress ring 108 shown in FIG. 3 develops when using crepe paper and aluminum foil materials. FIG. 4 illustrates an intermediate step in constructing the stress ring 108. Crepe paper insulation is wrapped around region 96 to form insulating region 98. Additional crepe paper and aluminum foil is wrapped on region 98 to form insulating region 102 and conducting region 104. The tubular member 106 is formed from crepe paper and tied around the foil region in 104. Insulation and foil regions 102 and 104, respectively, are peeled back around the tubular member 106. Crepe paper tape is wrapped around region 98 to form insulating region 100. This tape is then adjusted to conform to the shape of the tubular member to reduce voids in the solid insulation, such as the area 110. The crepe paper forming region 98 is peeled back and over the regions I00 and 102. The finished assembly resembles the stress ring 108 of FIG. 3.

An alternate stress ring arrangement is illustrated at FIG. 5. Insulating regions 96, 98, 100 and 102 are similar to those described concerning the typical stress ring 108 shown in FIG. 3. A tubular member 112 is constructed of an electrically conductive material, such as a metallic tube or an insulating tube with a conducting layer disposed thereon. The tubular member 112 is positioned'on or near a stress shield, represented here by conducting region 114. The potential stress at the end 116 of the conducting region 114 is transferred to the tubular member 112 which results in reducing the potential gradient of the combination.

A potential type instrument transformer constructed according to the teachings of this invention has smaller dimensions than a potential transformer constructed according to the prior art..For example, a 550 kv BIL potential transformer constructed according to the prior art has an overall height of 91 inches, contains 60 gallons of liquid dielectric and weighs 1950 pounds. A 550 kv BIL potential transformer constructed according to this invention has an overall height of 82 inches, contains 38 gallons of oil and weighs 1,205 pounds. It can be seen from this example that the invention disclosed herein may be used to appreciably reduce the size and weight of a potential transformer and subsequently improve manufacturing economies without sacrificing the rating or reliability of the transformer.

In summary, there has been disclosed a new and improved potential transformer utilizing a stress grading bushing assembly. The stress grading shields of the bushing assembly are terminated by circular stress rings which reduce the potential gradient at the ends of the stress shields. Reducing the potential gradient permits decreasing the insulation thickness and the overall transformer dimensions.

We claim:

1. A potential type instrument transformer comprising a metallic tank, a metallic tank cover having an opening therethrough, a magnetic core having high and low potential windings disposed thereon, solid insulation disposed around said windings, said core and windings being positioned in said metallic tank, a rigid insulating body member having an upper and a lower end and an opening extending between said ends, said insulating body member being mounted over said opening in said tank cover, a stress grading insulation assembly disposed in said opening of said insulating body member, an electrical conducting lead having two ends, said lead being disposed within said insulation assembly substantially coincident with the axis of said insulating body member, one end of said lead being connected to said high potential winding and the other end of said lead being connected to terminal means at said upper end of said insulating body member, said insulation assembly comprising a plurality of tubularlyshaped electrical conducting shielding members having upper and lower ends, said shielding members being disposed concentrically around said lead with the axes of said shielding members substantially coincident with said lead, said shielding members being separated from each other by solid electrical insulating material, said shielding member which is adjacent to said lead being substantially at the same potential as said lead and having the lower end of said shielding member below the outer surface of said solid insulation around said windings and adjacent to said high potential winding, said shielding member which is adjacent to said tank cover being substantially at the same potential as said tank cover and having said lower end of said shielding member closely adjacent to the plane containing said tank cover, said shielding members having their longitudinal dimension decrease as their radial distance from said lead increases, and stress relieving means positioned closely adjacent at least one end of each of said shielding members.

2. The potential type instrument transformer of claim 1 wherein the solid electrical insulating material comprises paper.

3. The potential type instrument transformer of claim 1 wherein the shielding member adjacent to the lead comprises a conducting material disposed on a tubular insulating member.

4. The potential type instrument transformer of claim 1 wherein the shielding member adjacent to the lead comprises a metallic tube.

5. The potential type instrument transformer of claim 1 wherein the shielding members other than the shielding member adjacent to the lead comprise a layer of conducting material disposed on the solid insulation.

6. The potential type instrument transformer of claim 1 wherein the stress relieving means comprises a ring-shaped member positioned adjacent to the end of the shielding member, said ring-shaped member effectively increasing the termination area of said shielding member, and said ring-shaped member being held in position by the solid insulating material covering said ring-shaped member.

7. The stress relieving means of claim 6 wherein the ring-shaped member comprises a conducting layer disposed on a tube of solid insulating material.

8. The stress relieving means of claim 6 wherein the ring-shaped member comprises a metallic tube or rod.

9. The stress relieving means of claim 6 wherein the shielding member is folded over the ring-shaped member. Y

10. A potential type instrument transformer comprising a metallic tank, a metallic tank cover having an opening therethrough, a magnetic core having high and low potential windings disposed thereon, solid electrical insulation disposed around said windings said core and windings being positioned in said tank, a rigid insulating body member having an upper and a lower end and an opening extending between said ends, said insulating body member being mounted over said openings in said tank cover, a stress grading insulation assembly disposed in said opening of said insulating body member, an electrical conducting lead having two ends, said lead being disposed within said insulation assembly substantially coincident with the axis of said insulating body member, one end of said lead being connected to said high potential winding and the other end of said lead being connected to terminal means at said upper end of said insulating body member, said insulation assembly comprising a plurality of tubularlyshaped electrical conducting shielding members having upper and lower ends, said shielding members being disposed concentrically around said lead with the axes of said shielding members substantially coincident with said lead, said shielding members being separated from each other by solid electrical insulating material, said shielding members having their longitudinal dimension decrease as their radial distance from said lead increases, a first of said shielding members being positioned adjacent to said lead and having substantially the same potential as said lead, said first shielding member having said lower end terminated by a ringshaped stress relieving member, said lower end of said first shielding member being positioned below the outer surface of said solid insulation around said windings and adjacent to said high potential winding, a second of said shielding members being positioned adjacent to said tank cover and having substantially the same potential as said tank cover, said second shielding member having said upper end and said lower end each terminated by a ring-shaped stress relieving member, said lower end of said second shielding member being positioned adjacent to the plane containing said tank cover, and a third of said shielding members being positioned between the other two said shielding members having its said upper end and said lower end each terminated by a ring-shaped stress relieving member, said lower end of said third shielding member being positioned adjacent to the insulation disposed on said high potential winding. 

1. A potential type instrument transformer comprising a metallic tank, a metallic tank cover having an opening therethrough, a magnetic core having high and low potential windings disposed thereon, solid insulation disposed around said windings, said core and windings being positioned in said metallic tank, a rigid insulating body member having an upper and a lower end and an opening extending between said ends, said insulating body member being mounted over said opening in said tank cover, a stress grading insulation assembly disposed in said opening of said insulating body member, an electrical conducting lead having two ends, said lead being disposed within said insulation assembly substantially coincident with the axis of said insulating body member, one end of said lead being connected to said high potential winding and the other end of said lead being connected to terminal means at said upper end of said insulating body member, said insulation assembly comprising a plurality of tubularly-shaped electrical conducting shielding members having upper and lower ends, said shielding members being disposed concentrically around said lead with the axes of said shielding members substantially coincident with said lead, said shielding members being separated from each other by solid electrical insulating material, said shielding member which is adjacent to said lead being substantially at the same potential as said lead and having the lower end of said shielding member below the outer surface of said solid insulation around said windings and adjacent to said high potential winding, said shielding member which is adjacent to said tank cover being substantially at the same potential as said tank cover and having said lower end of said shielding member closely adjacent to the plane containing said tank cover, said shielding members having their longitudinal dimension decrease as their radial distance from said lead increases, and stress relieving means positioned closely adjacent at least one end of each of said shielding members.
 2. The potential type instrument transformer of claim 1 wherein the solid electrical insulating material comprises paper.
 3. The potential type instrument transformer of claim 1 wherein the shielding member adjacent to the lead comprises a conducting material disposed on a tubular insulating member.
 4. The potential type instrument transformer of claim 1 wherein the shielding member adjacent to the lead comprises a metallic tube.
 5. The potential type instrument transformer of claim 1 wherein tHe shielding members other than the shielding member adjacent to the lead comprise a layer of conducting material disposed on the solid insulation.
 6. The potential type instrument transformer of claim 1 wherein the stress relieving means comprises a ring-shaped member positioned adjacent to the end of the shielding member, said ring-shaped member effectively increasing the termination area of said shielding member, and said ring-shaped member being held in position by the solid insulating material covering said ring-shaped member.
 7. The stress relieving means of claim 6 wherein the ring-shaped member comprises a conducting layer disposed on a tube of solid insulating material.
 8. The stress relieving means of claim 6 wherein the ring-shaped member comprises a metallic tube or rod.
 9. The stress relieving means of claim 6 wherein the shielding member is folded over the ring-shaped member.
 10. A potential type instrument transformer comprising a metallic tank, a metallic tank cover having an opening therethrough, a magnetic core having high and low potential windings disposed thereon, solid electrical insulation disposed around said windings said core and windings being positioned in said tank, a rigid insulating body member having an upper and a lower end and an opening extending between said ends, said insulating body member being mounted over said opening in said tank cover, a stress grading insulation assembly disposed in said opening of said insulating body member, an electrical conducting lead having two ends, said lead being disposed within said insulation assembly substantially coincident with the axis of said insulating body member, one end of said lead being connected to said high potential winding and the other end of said lead being connected to terminal means at said upper end of said insulating body member, said insulation assembly comprising a plurality of tubularly-shaped electrical conducting shielding members having upper and lower ends, said shielding members being disposed concentrically around said lead with the axes of said shielding members substantially coincident with said lead, said shielding members being separated from each other by solid electrical insulating material, said shielding members having their longitudinal dimension decrease as their radial distance from said lead increases, a first of said shielding members being positioned adjacent to said lead and having substantially the same potential as said lead, said first shielding member having said lower end terminated by a ring-shaped stress relieving member, said lower end of said first shielding member being positioned below the outer surface of said solid insulation around said windings and adjacent to said high potential winding, a second of said shielding members being positioned adjacent to said tank cover and having substantially the same potential as said tank cover, said second shielding member having said upper end and said lower end each terminated by a ring-shaped stress relieving member, said lower end of said second shielding member being positioned adjacent to the plane containing said tank cover, and a third of said shielding members being positioned between the other two said shielding members having its said upper end and said lower end each terminated by a ring-shaped stress relieving member, said lower end of said third shielding member being positioned adjacent to the insulation disposed on said high potential winding. 